Context and bypass encoding video

ABSTRACT

A method and apparatus for parallel context processing for example for high coding efficient entropy coding in HEVC. The method comprising retrieving syntax element relating to a block of an image, grouping at least two bins belonging to similar context based on the syntax element, and coding the grouped bins in parallel.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/518,407, filed Jul. 22, 2019, which is a continuation of U.S. patentapplication Ser. No. 15/295,689, filed Oct. 17, 2016 (now U.S. Pat. No.10,362,322), which is a continuation of U.S. patent application Ser. No.13/184,226, filed Jul. 15, 2011 (now U.S. Pat. No. 9,591,320), whichclaims the benefit of U.S. Provisional Application No. 61/499,852, filedJun. 22, 2011, and claims the benefit of U.S. Provisional ApplicationNo. 61/364,593, filed Jul. 15, 2010, the entireties of all of which arehereby incorporated by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

Embodiments of the present invention generally relate to a method andapparatus for parallel context processing techniques for high codingefficiency entropy coding, which may be used in the video codingstandard High Efficiency Video Coding (HEVC).

Description of the Related Art

Context-Adaptive Binary Arithmetic Coding (CABAC) is one of two entropyengines used by the existing video coding standard AVC. CABAC is amethod of entropy coding that provides high coding efficiency.Processing in CABAC engine is highly serial in nature. Consequently, inorder to decode high bit rate video bit-streams in real-time, the CABACengine needs to be run at extremely high frequencies which consumes asignificant amount of power and in the worst case may not be feasible.

Therefore, there is a need for an improved method and/or apparatus forparallel context processing techniques for high coding efficiencyentropy coding in HEVC.

SUMMARY OF THE INVENTION

Embodiments of the present invention relate to a method and apparatusfor parallel context processing for example for high coding efficiententropy coding, such as, HEVC. The method comprising retrieving syntaxelement relating to a block of an image, grouping at least two binsbelonging to similar context based on the syntax element, and coding thegrouped bins in parallel.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is an embodiment of a CABAC block diagram;

FIG. 2 is an embodiment of a flow diagram depicting PIPE/V2V coding;

FIG. 3 is an embodiment of a syntax element partitioning;

FIG. 4A is an embodiment of a flow diagram depicting a parallelizationof context processing for significance map utilizing speculativecomputing at each bin;

FIG. 4B is an embodiment of a flow diagram depicting a parallelizationof context processing for significance map utilizing speculativecomputing at a fifth bin;

FIG. 5 is an embodiment of a flow diagram depicting a method for contextprocessing tree for level coding in AVC;

FIG. 6 is an embodiment of a flow diagram depicting context processingtree for levels when SIGN is coded in separate bin-plane; and

FIG. 7 is an embodiment of a proposed approach on order of syntaxelements.

DETAILED DESCRIPTION

FIG. 1 is an embodiment of a CABAC block diagram. As shown in FIG. 1,the serial nature in CABAC comes from the following three blocks, abinarizer, a context modeler and a binary arithmetic coder. In thebinarizer, bins from many syntax elements, such as, coefficient levelsand motion vector differences are coded using variable length codingsuch as unary coding and exp-Golomb coding. Variable length codes areinherently serial in nature. In the context modeler, the serialdependency comes about since the probability used in the context modelfor coding the next bin is updated depending on the current bin value.If the current bin value is Least Probable Symbol (LPS), the probabilityis increased and if the current bin value is Most Probable Symbol (MPS),the probability is decreased. Another source of serial dependency is thecontext index selection process, where the context index of bin may bedetermined by the value of previously coded bins. In the binaryarithmetic coder (BAC), the arithmetic coding uses interval subdivision.The range, value, offset used to determine the interval on [0, 1] thatuniquely identifies the coded stream of bin values are updated in aserial fashion as and when bins get encoded/decoded.

In some embodiments of parallel entropy coding tools, the parallelismproposed may be broadly classified into three categories: (1) Bin-levelparallelism, which parallelizes the BAC, (2) Syntax element-levelparallelism, which parallelizes the BAC, the context modeler, and thebinarizer and (3) Slice-level parallelism.

A N-bins/cycle coding (NBAC) encodes and decodes N-bins/cycle to achieveN-fold improvement in throughput. The contexts for N-bins are calculatedthrough the use of conditional probabilities. In some HEVC embodiment,the binarizer and context modeler were basically the same as in CABAC ofAVC. However, coding schemes are determined variable-to-variable lengthfor coding of the bins. There are two flavors of the scheme: (1) PIPEand (2) V2V. The main difference between the two is the contextprobabilities are quantized to 12 levels in PIPE and to 64 in V2V. InPIPE/V2V coding scheme, the bins are coded using a parallel bin encodingscheme as shown in FIG. 2. FIG. 2 is an embodiment of a flow diagramdepicting PIPE/V2V coding.

Some embodiments that utilize schemes that interleaves the V2V codewords from different partial bitstreams into a single bitstream. As aresult, a throughput increase of 6X for PIPE in hardware is possible.Such embodiments usually cause an estimated throughput increase of 3X inBAC stage for PIPE hardware implementation for both the parallel andserial versions of PIPE. Since PIPE uses 12 bitstream buffers and V2Vuses 64 bitstream buffers, PIPE is usually utilized more often than V2Vfrom a complexity purpose. However in both cases, there is no estimatedoverall throughput improvement in the entropy coder due to serialbottlenecks in context processing and binarization.

The NBAC, PIPE, V2V schemes reduces serial dependency in the BAC block.However, the serial dependency in the context modeler and binarizerstill remain. So, the effective throughput increase that can be achievedin entropy coding is limited. Hence, techniques for parallelization ofcontext processing (PCP) may be utilized.

In syntax element partitioning, syntax elements such as macroblock type,motion vectors, transform coefficients, significant coefficient map etc.are divided into N groups and each group is coded separately. Thecontext selection and adaptation within a group happens in parallelleading to a potential N-fold speed up in context modeler if the variouspartitions are balanced in terms of the number of bins they process. Inpractice, the various partitions are not balanced and the throughputimprovement is less than a factor of N.

FIG. 3 is an embodiment of a syntax element partitioning. FIG. 3 showsthe block diagram of a system with N syntax partitions. The bin coderscan be arithmetic coders or PIPE/V2V coders. If PIPE/V2V coders are usedas the bin coders, the serial version of PIPE interleaving codewordsmaybe preferable for reducing the number of bitstream buffers.

Syntax element partitioning results showed throughput improvement andBD-Rate. In this embodiment, significance map coding is carried out inAVC CABAC. In such an embodiment, the last significant coefficient flagis transmitted when the related coefficient is determined to besignificant. The coefficient is the output of a block after transformand quantization. Also, a coefficient is significant when it has valuethat is non-zero.

This technique introduces serial dependency in decoding of significancemap. When throughput improvement is needed, speculative computation areperformed at every bin. Such computations leads to complex logic, asshown in FIG. 4A. FIG. 4A is an embodiment of a flow diagram depicting aparallelization of context processing for significance map utilizingspeculative computing at each bin. Speculative computation at every binalso results in increased power consumption.

Significance map coding are parallelized by transmitting the lastsignificant coefficient flag once per certain number of bins. Forexample, FIG. 4B is an embodiment of a flow diagram depicting aparallelization of context processing for significance map utilizingspeculative computing at a fifth bin. If all of the significantcoefficient flag is zero, then the last significant coefficient flag isnot transmitted.

Such an embodiment reduces the number of last bins that need to betransmitted, but it increases the number of significant bins that needto be transmitted. However, there is about a 5% overall reduction in thenumber of significance map bins that need to be processed. Our algorithmparallelizes about 21.65% of the bins for largest coding unit (LCTB).

Table 1 shows the distribution of bins used by different syntax elementtypes as a percent of total bins for a LCTB. The bin distribution wasobtained by measuring bins in bitstreams generated, for example, byTMuC-0.1 using cfg files in cfp-fast directory. Shown in Table 1 is thedistribution of bins used by different syntax element type as a percentof total bins for a LCU.

TABLE 1 Bins used Average per number syntax of bins SigMap 21.65%SigLast  8.35% LevelAbs 16.67% LevelSign  9.92%The coefficient coding is usually carried out in AVC CABAC. The contextused for the absolute value of the coefficient minus one, known as thecoefficient level (1) depends on the position of the bin. Thus, when thebinldx is 0 (i.e. first bin of the coefficient level), then the contextis derived by (ctxldxlnc=((numDecodAbsLevelGt1!=0)?0: Min(4,1+numDecodAbsLevelEq1))); Otherwise, context is devided by(ctxldxlnc=5+Min(4−((ctxBlockCat==3)?1:0), numDecodAbsLevelGt1)).Context processing for the first bin in the absolute value of thecoefficient minus one (i.e. Coeff Level BinIdx 0 in FIG. 7) is differentfrom the other bins in the coefficient level.

In one embodiment, the encoding Coeff Level BinIdx 0 occurs in aseparate bin-plane as shown in the second row of FIG. 7. The advantagein the context processing, because it can be carried out in parallel tothe rest of the context processing i.e. the context processing for allthe Coeff Level BinIdx 0, for all the coefficients level in a block, maybe carried out in parallel to bin processing of Coeff Level BinIdx 0before the decoding of the other bins in the coefficient level. This isreferred to as Coeff Level BinIdx PCP.

In AVC, sign information is interleaved along with level information asshown in FIG. 5. This leads to inefficiency in parallel contextprocessing. FIG. 5 is an embodiment of a flow diagram depicting a methodfor context processing tree for level coding in AVC. In FIG. 5, thecontext processing tree that needs to be pre-calculated at each bin toachieve 4X parallelism in context processing of level in AVC. Thecontext processing that happens at every SIGN node is wasteful sinceSIGN is coded in bypass mode. Table 3 shows the distribution of level ofcoefficients obtained by measuring levels in bitstreams generated by,for example, a TMuC-0.1 using cfg files in cfp-fast directory.

TABLE 3 Probablity of Level occurrence 1 0.76 2 0.15 3 0.05 4 0.02 50.01

Level=1 occurs with the highest probability, so the most probable pathin the context processing tree of FIG. 6 is L0(0)→SIGNO→L1(0)→SIGN1. Forthis particular path, the context processing efficiency is 50%, meaninghalf the context processing is wasteful. On the average, for the contextprocessing tree of FIG. 6 and assuming the level distribution of Table3, the context processing efficiency is 60%. FIG. 6 is an embodiment ofa flow diagram depicting context processing tree for levels when SIGN iscoded in separate bin-plane. In FIG. 6, the context processing tree forlevels when sign is coded in separate bin-plane. As can be seen in thefigure, context processing efficiency is 100%. This is also illustratedin FIG. 7 where all sign bins (i.e. Coeff Sign Bins) are coded onseparate bin plans; this is referred to as Coeff Sign PCP.

In some embodiment, the first two bins in the coefficient level arecontext coded. The rest of the bins, such as, coefficient sign bins andGolomb−Rice+Exp−Golomb (GR-EG) binarized bins, are bypass coded. As anextension of “Coeff Level BinIdx 0 PCP”, the second bin in the absolutevalue of the coefficient minus 1 (i.e. Coeff Level BinIdx 1) is alsocoded in a separate bin-plane. The Coeff Sign Level can be interleavedor be on a separate bin-plane with GR-EG bins.

FIG. 7 is an embodiment of a proposed approach on order of syntaxelements. FIG. 7 illustrates a data ordering based on Coeff Level BinIdx0 PCP, Coefficient Sign PCP, and Coeff Level BinIdx 1 PCP. Bypass codedbins are Coefficient Sign & GR-EG bins. The first row shows originalordering used in H.264/AVC. The ordering of HEVC (HM−1.0), in which theproposed Coeff Level BinIdx 0 PCP and Coefficient Sign PCP was adopted,is shown in the second row. Here c0 and sign can be placed in partitionsthat can be coded in parallel with the other bins. The new coefficientlevel binarization and coding introduced in HM−3.0 is shown in the thirdrow. Finally, the proposed Coeff Level BinIdx 1 PCP is shown in thefourth row. Here c0, c1 and sign+GR-EG bins can be placed in partitionsthat can be coded in parallel with the other bins. Note that sign andGR-EG bins (i.e. exp-golomb and golomb rice bins of coeff) can be placedin the same partition as all are bypass coded.

Since bypass coding is simpler than context coding, bypass bins can becoded faster than context coded bins. In particular, many bypass binscan be coded in a cycle which can increase the throughput of the CABAC.With Coeff Level BinIdx 1 PCP all bypass coded bins for coefficients ina given TU are grouped together which increases throughput impact ofparallel bypass bins processing.

Variants of this approach include separating GR-EG+sign bins from theCoeff Level BinIdx 0 and Coeff Level BinIdx 1, but keeping theGR-EG+sign bins interleaved and keeping the Coeff Level BinIdx 0 andCoeff Level BinIdx 1 bins interleaved as shown in proposal #2 in FIG.11. This eliminates the additional of loops required to separate theCoeff Level BinIdx 0 from Coeff Level BinIdx 1, and Coefficient Signfrom RG-EG bins. Alternatively, proposal #3 in FIG. 7 keeps GR-EG andCoefficient sign bins interleaved, to reduce the loops, and keeps CoeffLevel BinIdx 0 and Coeff Level BinIdx 1 in separate partitions. This isdue to the fact that the context selection for Coeff Level BinIdx 0 andCoeff Level BinIdx 1 are more complex and keeping the two separate helpsto improve parallel processing as described in section Coeff LevelBinIdx 0 PCP. This approach can also be applied to other type of syntaxelements such as motion vector difference. The bypass bins of the motionvectors difference can be coded together on a separate bin-plane thanthe context coded bins.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

What is claimed is:
 1. A method comprising: receiving a stream ofencoded bits; extracting a first context-encoded syntax element from theencoded bits, the first context-encoded syntax element related to anabsolute value of a first significant transform coefficient level in ablock; following the extraction of the first context-encoded syntaxelement, extracting a second context-encoded syntax element from theencoded bits, the second context-encoded syntax element related to anabsolute value of a second significant transform coefficient level inthe block; following the extraction of the second context-encoded syntaxelement, extracting a first bypass-encoded syntax element from theencoded bits, the first bypass-encoded syntax element related to a signof the first significant transform coefficient level in the block;following the extraction of the first bypass-encoded syntax element,extracting a second bypass-encoded syntax element from the encoded bits,the second bypass-encoded syntax element related to a sign of the secondsignificant transform coefficient level in the block.
 2. The method ofclaim 1, wherein: extracting the first context-encoded syntax elementfrom the encoded bits further comprises decoding the firstcontext-encoded syntax element using context processing; extracting thesecond context-encoded syntax element from the encoded bits furthercomprises decoding the second context-encoded syntax element usingcontext processing; extracting the first bypass-encoded syntax elementfrom the encoded bits further comprises decoding the firstbypass-encoded syntax element using bypass mode; and extracting thesecond bypass-encoded syntax element from the encoded bits furthercomprises decoding the second bypass-encoded syntax element using bypassmode.
 3. The method of claim 1, further comprising: extracting a firstsignificant coefficient flag, the first significant coefficient flagindicating a significant coefficient at a first position in the block;and extracting a second significant coefficient flag, the secondsignificant coefficient flag indicating a second position in the block;and wherein: the first context-encoded syntax element corresponds to theabsolute value of the first significant transform coefficient level atthe first position in the block; and the second context-encoded syntaxelement corresponds to the absolute value of the second significanttransform coefficient level at the second position in the block.
 4. Themethod of claim 3, wherein: extracting the first context-encoded syntaxelement from the encoded bits comprises extracting the firstcontext-encoded syntax element from the encoded bits in response to avalue of the first significant coefficient flag; and extracting thesecond context-encoded syntax element from the encoded bits comprisesextracting the second context-encoded syntax element from the encodedbits in response to a value of the second significant coefficient flag.5. The method of claim 4, wherein: extracting the first bypass-encodedsyntax element from the encoded bits comprises extracting the firstbypass-encoded syntax element from the encoded bits in response to avalue of the first significant coefficient flag; and extracting thesecond bypass-encoded syntax element from the encoded bits comprisesextracting the second bypass-encoded syntax element from the encodedbits in response to a value of the second significant coefficient flag.